Electromagnetic reciprocating compressor with spring assembly mounted around piston

ABSTRACT

An electromagnetic reciprocating compressor has a body (1, 2), a piston (10) reciprocating in the body, and an electromagnetic drive (22, 23, 24) for the piston. The piston has a piston head (11) which slides in a cylinder (12) in the body to effect compression of fluid in the cylinder during operation of the compressor and, axially spaced from the piston head, a piston guide member (13) slidingly movable on a guide surface (14) provided by the body. The compressor has a compression spring arrangement (20) comprising at least one helical compression spring (20a, b) acting to drive the piston axially. The spring (20a, b) is mounted around the piston and is at least partly located within the cylinder during at least part of the piston stroke but is outside the working volume of fluid undergoing compression.

TECHNICAL FIELD

This invention relates to electromagnetic reciprocating compressors orpumps, particularly compressors for pumping gas such as air. Suchdevices can also act as vacuum pumps, but the term "compressor" will beused generally in this specification for convenience.

BACKGROUND TO THE INVENTION

International patent applications PCT/GB94/01193, PCT/GB94/01194,PCT/GB94/01195, published as WO 94/28306, WO 94/28307 and WO 94/28308respectively, and deriving from one of the inventors of the presentapplication, describe a compressor which provides a number ofimprovements in the art. Reference should be made to these applications,hereinafter called "the earlier applications", for background discussionof such compressors and for details of the compressor described in thoseapplications.

The present invention seeks to provide some modification and improvementof the compressor of the earlier applications. Particularly the presentinvention is concerned with the problem of reducing wear, particularlyuneven wear, of the sliding surfaces of the reciprocating piston. Inaddition the invention is concerned with improving the ease ofmanufacture and assembly of the compressor.

It is well known in electromagnetic reciprocating compressors to providea helical compression spring which causes the return stroke of thepiston. In the earlier applications, particularly WO 94/28306, it isdescribed how inherent defects in the compression spring or misalignmentin its mounting can cause the spring to apply unsymmetric force to thepiston, resulting in uneven wear.

It is known from GB-A-2241287 in a double-acting electromagneticcompressor, in which the piston alternately compresses gas in twoopposed working chambers, to mount two compression springs which restorethe piston to a neutral position inside the respective working chambers.

It is known from EP-A-509660 to locate a return spring around the pistonand partly in the cylinder of the compressor outside the working chamberof the cylinder.

SUMMARY OF THE INVENTION

According to the present invention in one aspect there is provided anelectromagnetic reciprocating compressor having a body, a pistonreciprocating in the body, and an electromagnetic drive for the piston,the piston having a piston head which slides in a cylinder in the bodyto effect compression of fluid in the cylinder during operation of thecompressor and, axially spaced from the piston head, a piston guidemember slidingly movable on a piston guide surface provided by the body,the piston carrying an armature forming part of said electromagneticdrive, the compressor having a compression spring arrangement comprisingat least one helical compression spring acting to drive the pistonaxially, which spring is mounted around the piston and is at leastpartly located within the cylinder during at least part of the pistonstroke but is outside the working volume of fluid undergoingcompression, characterised in that, for replacement or maintenance, thepiston including the armature and the piston guide member is removablefrom the body through the end of the cylinder remote from the pistonguide surface. Typically therefore the compression spring acts at oneend on a surface of the piston which is outside the working chamber, inwhich the fluid is compressed.

The invention is especially applicable to a single-acting compressor,i.e. one in which there is only one working chamber in which fluid iscompressed by the action of the piston and the piston has a singlepiston head surface which faces the working chamber.

Preferably the compression spring acts at one end upon a rear face ofthe piston head.

Preferably the or each compression spring is freely rotatable at atleast one end relative to the piston. There may be a single compressionspring, but preferably two such springs are employed in the springarrangement, with at least one of the springs being at least partly inthe cylinder. Most preferably there are a pair of helical springs ofopposite helical coiling sense acting in series and with a free bearingmember between them, as described in the earlier applications. In thepresent invention this free bearing member may be an annular membersurrounding the piston with clearance from the piston.

Preferably, as in the earlier applications, each helical spring ismounted at one end on the bearing so as to be rotatable relative to thebearing, for example by the interposition of a low friction material.

The opposite end of the spring arrangement from the end acting upon thepiston head may be located on a mounting member which bears on alamination stack providing a stator of the electromagnetic drive of thecompressor.

Advantages which can be provided by the invention in this first aspectare a reduced overall length of the compressor, because the returnspring for the piston is not provided between the piston and a rear endof the compressor body as is conventional. The helical spring or springswhich surround the piston, can be of larger diameter than isconventional, with the result that the spring is more stable and is lesslikely to exert a lateral force on the piston, due to lateral flexing ofthe spring. Suitably the exterior diameter of the helical spring in thecylinder is at least 70%, more preferably at least 80% of the internaldiameter of the cylinder. It is also thought that direct application ofthe force of the compression spring to the piston head may reducelateral forces on the piston head. Consequently, there is less unevenwear of the piston head and the cylinder surface against which itslides. These effects are improved, by the use of two helical springs ofopposite helical coiling sense.

In another aspect, the present invention provides an electromagneticreciprocating compressor having a body, a piston reciprocating in thebody and an electromagnetic drive for the piston, the body having frontand rear body parts separated from each other by a stack of magneticallypermeable laminations providing a stator of the electromagnetic drive,the front and rear body parts and preferably also the lamination stackbeing located and aligned relative to each other by locating pinsreceived in locating holes in the body parts and passing through thestack of laminations. The locating pins preferably extend axially.

The locating pins may be dowel pins of C-section which are resilientlycompressible for insertion in the locating holes.

Preferably the front body part provides a cylinder surface on which apiston head of the piston slides, and the rear body part provides apiston guide surface or surfaces against which a piston guide member ofthe piston slides. Each of the cylinder surface and the piston guidesurface or surfaces is preferably machined to its final size withreference to at least one of the locating holes, so that in theassembled compressor, the cylinder surface and the piston guide surfaceor surfaces are accurately aligned.

The front and rear body parts may be secured together by bolts.

The invention in this aspect can provide good alignment of the parts ofthe compressor, particularly front and rear body parts and thelamination stack. The good alignment of the body parts provides accuratealignment of the surfaces on which the piston slides, as describedabove. Accurate location, in the radial direction, of the laminationstack can allow the air gap between the interior surface of thelamination stack and the exterior surface of an armature on the pistonto be small, which leads to higher electrical efficiency and thereforelower power consumption by the compressor. This air gap may be below0.5mm and even as low as 0.1 mm. As illustrated by the specificembodiment below, the construction provided by this aspect of theinvention can allow the front and rear body parts to be made fromidentical base castings, these base castings being subjected tomachining operations to provide the desired final shapes of the frontand rear body parts. This simplifies the manufacture of the compressor.

DESCRIPTION OF AN EMBODIMENT

One embodiment of the invention will now be described by way ofnon-limitative example with reference to the accompanying drawings, inwhich:

FIG 1 is an axial section of a compressor embodying the invention, onthe line I--I of FIG. 2; and

FIG. 2 is an axial section of the compressor of FIG. 1, on the lineII--II of Fig 1.

The compressor shown in the drawings is generally similar in operationto the compressor shown in the earlier applications, and parts havingthe same function are given the same reference numerals as in theearlier applications.

The compressor shown in the present drawings has a front body part 1 ofsquare exterior cross-section transverse to the axis and a rear bodypart 2 also of square exterior cross-section secured together by bolts 4(see FIG. 2), with electrically insulating washers 5 provided in pairsunder the bolt heads, to avoid electrical connection of the body parts1, 2 to each other. A cylinder head 3 is secured to the front body part,and closes a circumferential recess 62 in the front body part whichprovides a buffer volume for the air compressed by the compressor tosmooth the flow, and connects to an outlet connector 3a. Similarly, atthe rear end, the rear body part 2 has an end plate 8 which includes anair inlet connector 8a. The incoming air passes through a filter 7 in arecess in the rear part body 2 which corresponds in shape to the recess62 in the front part body, for reasons explained below. The piston head3 and the end plate 8 are secured to the body parts 1, 2 by bolts (notshown).

Axially reciprocatingly movable within the compressor is a piston 10having a piston head 11 which has a peripheral continuous band 15 ofplastics material moulded onto it and sliding on a cylinder surface 12provided by the front body part 1. At the rear end of its part providingthe piston head 11, the piston 10 has an armature 24, and rearwardly ofthat a rear piston guide member 13, these parts being secured togetherby a bolt 16. The front portion 13a of the rear piston guide member 13is in one piece with the cylindrical portion 13b which carries at itsexternal periphery a moulded continuous band of low friction plasticsmaterial 13c, which slides on part-cylindrical piston guide surfaces 14provided by the rear body part 2. The electromagnetic linear drive ofthe compressor is provided by the armature 24 together with coils 22 anda stack 23 of magnetically permeable laminations interposed between thebody parts 1, 2 and forming a stator. This linear drive is ofconventional type and need not be described further, and drives thepiston 10 in one direction (to the right in FIGS. 1 and 2). The reverse(compression) stroke of the piston 10 is caused by the spring system 20described below.

The piston head 11 has air flow passages 30, which may be inclined tothe axis of the compressor, as described in the earlier applications, toprovide a rotating force to the piston 10 during operation, by turbineeffect. These apertures 30 are closed at the face of the piston head 11by a flexible sheet 31 which provides a flap valve over each aperture30. Through the wall of the cylinder there is a bore 32 for outflow ofcompressed air, also closed by a flap valve (not shown) at its outletend.

The two body parts 1, 2, and also the stator 23, are accurately alignedrelative to each other against relative radial displacement, by a pairof dowel pins 50 located at opposite sides of the compressor body andtightly held in opposed blind bores 51 in the body parts 1, 2. The dowelpins 50 have a C-shape in cross-section and are made of spring steel sothat they may be easily inserted in the bores 51 but after insertionopen to hold tightly in the bores 51 and provide accurate alignment ofthe two body parts 1, 2 relative to each other. At the same time, thepins 50 pass through holes in the stack 23 of laminations which areapproximately the same size as the pins 50, so that the whole of thestack 23 is also accurately radially located. Because of this accuratelocation of the stack 23 of laminations, the air gap between theinterior face of the stack 23 and the armature 24, seen in FIG. 1, canbe minimised and may be as small as 0.1 mm.

The body parts 1, 2 are made from identical castings, in aluminium.After casting, the parts are machined to provide the two bores 51 onaccurately spaced axes, and thereafter the cylindrical surface 12 of thefront body part 1 and the cylindrical surface 14 of the rear body part 1are formed and machined to the desired diameters and on axes which areaccurately located relative to the bores 51. When the body parts 1, 2and the stack 23 are assembled together by means of the pins 50 andbolts 4, the cylindrical surfaces 12, 14 are very accurately coaxiallyaligned, so that the piston 10, which is subsequently inserted beforefitting of the cylinder head 3, is itself accurately aligned in thecompressor. Consequently, wear in the compressor, and in particularuneven wear, which might result from misalignment of the surfaces 12, 14is minimised.

To avoid a shorted turn in the structure of the compressor, the bodyparts 1, 2 are electrically isolated from each other. For this purpose,the surfaces of the body parts are anodised, so that there is noelectrical connection between them via the pins 50 or through the stack23.

The spring system 20 providing the return stroke of the piston 10 hastwo springs 20a and 20b, which are helical coil springs of mutuallyopposite direction of coiling, i.e. one of the springs is a right-handhelix and the other is a left-hand helix. Between the opposed ends ofthese springs is a bearing 40 which is an annular body surrounding thepiston 10 with a clearance, so that it is supported only by the springs20a, 20b and free to move relative to the piston 10. The bearing 40provides seats for the ends of the springs 20a and 20b on rings 42 oflow friction material such as PPS (polyphenylene sulphide) blended witha percentage of a lubricating medium and a percentage of reinforcingfibre. These rings 42 lie on opposite sides of a radial flange 41 of thebearing 40 which has a cylindrical sleeve portion 43, which locates theends of the springs radially. Both springs 20a, 20b can thus rotate atone end essentially freely relative to the bearing 40 (and relative tothe piston and each other) about their central axis. The other end ofthe spring 20a bears on a rear face of the piston head 11 through aflange locating ring 44 which locates the axis of the spring relative tothe piston 10. Similarly, the other end of the spring 20b is located bya flanged ring 45 which is radially located in the body part 1 as shownin FIG. 2 and abuts axially on the stack 23 of laminations. This end ofthe spring 20b is also radially fixed in this manner.

It can be seen that the spring 20a is partly within the cylinder surface12, during at least part of the stroke of the piston. The exteriordiameter of the springs 20a, 20b is about 85% of the diameter of thecylinder surface 12. The whole of the spring system surrounds the piston10 between the piston head 11 and the rear piston 13. In thisembodiment, the springs 20a and 20b have a larger diameter than the rearpiston 13, to enable assembly of the device and also a larger diameterthan the armature 24. Likewise the bearing 40 has a larger diameter thanthe rear piston 13 and the armature 24.

The ability of the springs 20a and 20b to rotate freely at one endrelative to each other and relative to the piston 10 and the body of thecompressor means that they do not tend to exert a rotational torque onthe piston and also do not tend to distort laterally, so as to applylateral force to the piston 10. The springs are of relatively largediameter and therefore are more stable against lateral distortion thannarrower diameter springs.

We claim:
 1. An electromagnetic reciprocating compressor comprising abody, a piston reciprocating in the body, and an electromagnetic drivefor the piston, the piston having a piston head which slides in acylinder in the body to effect compression of fluid in the cylinderduring operation of the compressor and, axially spaced from the pistonhead, a piston guide member slidingly movable on a piston guide surfaceprovided by the body, the piston carrying an armature forming part ofsaid electromagnetic drive and said body including a lamination stackproviding a stator of said electromagnetic drive; andthe compressorfurther comprising a compression spring arrangement comprising at leastone helical compression spring acting to drive the piston axially, whichsaid spring is mounted around the piston and is at least partly locatedwithin the cylinder during at least part of a piston stroke but isoutside a working volume of fluid undergoing compression; and wherein,for replacement or maintenance, the piston including the armature andthe piston guide member is removable from the body through an end of thecylinder remote from the piston guide surface, and wherein said springarrangement is located at one end on a mounting member which bears onsaid lamination stack providing said stator.
 2. A compressor accordingto claim 1 wherein said compression spring acts at one end upon a faceof the piston head which is outside the working volume of fluidundergoing compression.
 3. A compressor according to claim 1 whereinsaid compression spring is freely rotatable at at least one end relativeto the piston.
 4. A compressor according to claim 1 wherein saidcompression spring arrangement comprises at least two compressionsprings mounted around the piston, at least one said spring being atleast partly in said cylinder.
 5. A compressor according to claim 4wherein said compression spring arrangement has a pair of helicalsprings of opposite helical coiling sense acting in series, and abearing between opposed ends of said pair of helical springs permittingrelative rotation of those ends, the bearing being freely movablerelative to the piston and the cylinder.
 6. An electromagneticreciprocating compressor comprising a body, a piston reciprocating in anaxial direction in the body and an electromagnetic drive for the piston,the body having front and rear body parts separated from each other by astack of magnetically permeable laminations providing a stator of saidelectromagnetic drive, the front and rear body parts being located andaligned relative to each other by resiliently compressible locating pinstightly received in locating holes in the body parts and passing throughthe stack of laminations thereby locating the stack of laminationsradially.
 7. A compressor according to claim 6 wherein said front bodypart provides a cylinder surface on which a piston head of the pistonslides, and said rear body part provides at least one piston guidesurface against which a piston guide member of the piston slides, andeach of said cylinder surface and said at least one piston guide surfaceis machined to its final size with reference to at least one of thelocating holes.
 8. An electromagnetic reciprocating compressorcomprising a body, a piston reciprocating in the body, and anelectromagnetic drive for the piston, the piston having a piston headwhich slides in a cylinder in the body to effect compression of fluid inthe cylinder during operation of the compressor and, axially spaced fromthe piston head, a piston guide surface provided by the body, the pistoncarrying an armature forming part of said electromagnetic drive;thecompressor further comprising a compression spring arrangementcomprising at least two helical compression springs acting to drive thepiston axially, which said springs are mounted around the piston and atleast one said spring is at least partly located within the cylinderduring at least part of a piston stroke but is outside a working volumeof fluid undergoing compression; and wherein, for replacement ormaintenance, the piston including the armature and the piston guidemember is removable from the body through an end of the cylinder remotefrom the piston guide surface.
 9. A compressor according to claim 8wherein said compression spring arrangement has a pair of helicalsprings of opposite helical coiling sense acting in series, and abearing between opposed ends of said pair of helical springs permittingrelative rotation of those ends, the bearing being freely movablerelative to the piston and the cylinder.